An advantage of light-emitting diode (LED) arrays, which comprise a plurality of p-n junction light-emitting diodes arrayed on a substrate, is that they enable image information and the like to be processed relatively easily by electrically controlling the individual light-emitting diodes. As such, improved LED arrays have been proposed, together with various applications.
One application for such LED arrays is in printers used as information output devices. There is a need for printers that can print faster and produce higher density output. One way of meeting this need is to use light-emitting diode arrays as a light source. Laser printers, which employ a laser light source, and LED printers, which employ an LED array as the light source, are two examples of non-impact optical printers. A laser printer requires the use of a mechanical mechanism such as a rotating polygonal mirror for the scanning laser beam, and a correspondingly complex optical system. An LED printer, on the other hand, only requires that the light-emitting diodes that constitute the LED array are controlled to switch them on and off. This enables LED printers to be smaller, faster and more reliable than laser printers.
FIG. 1 is a cross-sectional view of a conventional (prior art) LED 10 for use in an LED printer or the like. LED 10 comprises a substrate 11 of n-type conductivity GaAs having on a surface thereof a layer 19 of n-type conductivity GaAsP. The layer 19 may be formed on the substrate 11 by vapor-phase epitaxy (VPE). On the layer 19 is a masking layer 15 of SiN having an opening therethrough to expose a portion of the layer 19. A p-type conductivity diffusion region 18 extends into the layer 19 under the opening in the masking layer 15. The diffusion region 18 is formed by diffusing a p-type conductivity dopant, such as zinc, into the layer 19 through the opening in the masking layer 15. A light emission region is formed by the p-n junction formed by the interface between the layer 19 and the diffusion region 18. A conductive electrode 16 is on a surface of the diffusion region 18 within the opening in the masking layer 15, and a conductive electrode 17 is on a surface of the substrate 11 opposite the layer 19.
A problem with this type of surface emitting type LED is that most of the light is produced below the electrode 16 where the current density is at its highest. However, this light is reflected by the electrode 16 and therefore is not emitted externally. This greatly decreases the external quantum efficiency of the LED 10.